die quickly, while the guinea-pig which furnished the
virus, recovers without the least suffering. We are
present here, then, at a restricted evolution of a micro-
scopic organism which causes the formation of pus and
a closed abscess without bringing about any internal
disturbance or the death of the animal on which it
occurs, and, nevertheless, one which is always ready to
convey death to other species into which it is inoculated,
ready even to kill the animal on which it occurs in the
form of an abscess, if more or less fortuitous circum-
stances enable it to pass into the blood or into the
splanchnic organs.

"Chickens or rabbits which live in the company of
guinea-pigs bearing such abscesses may suddenly become
sick and die without the health of the guinea-pigs appear-
ing to be in the least impaired. For this to occur, it
is only necessary that the abscesses of the guinea-pigs
should rupture, scattering a little of their contents on
the food of the chickens and the rabbits. An observ-
ing person, seeing these facts and ignorant of the rela-
tion of which I am speaking, would be astonished to see
chickens and rabbits destroyed without any apparent
cause, and would believe that the disease was sponta-
neous, for he would be far from supposing that it had
originated in the guinea-pigs, all in good health, especially
if he knew that the latter are also subject to the same
disease. How many mysteries in the history of conta-
gions will some day receive solutions still more simple
than that of which I have just been speaking! Let
us reject theories which we can contradict by convincing
facts, but not on the vain pretext that certain of
their applications escape us. The combinations of
nature are at the same time more simple and more
varied than those of our imagination!"

For anyone who pondered over Jenner's work, what was

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